The Freedom from Religion Foundation has filed a lawsuit to stop Texas Gov. Rick Perry from holding a day of prayer event in Houston.
In a video promoting the Aug. 6 event dubbed "The Response," Perry called on Americans to "make plans to be part of something even bigger than Texas."
But the watchdog group of atheists and agnostics said the governor's call for a day of prayer violates the constitutional ban on the government establishing a religion.
The plaintiffs are "nonbelievers who support the free exercise of religion, but strongly oppose the governmental establishment and endorsement of religion, including prayer and fasting, which are not only an ineffectual use of time and government resources, but which can be harmful or counterproductive as a substitute for reasoned action," the group said in their complaint.
Eric Bearse, spokesman for The Response, told ABC News the lawsuit, came as no surprise.
"We expected this kind of legal harassment," he said.
"But the right of Americans to assemble and pray has been established for over 200 years," he added. "We are confident we will be victorious."
Gov. Perry has invited the president, the nation's governors, and Texas lawmakers to attend the Houston prayer meeting. So far, more than 6,000 people have registered for the event.